What is the principle behind 'activated sludge' in a biological sewage treatment plant?

The principle behind activated sludge in a biological sewage treatment plant is to use a concentrated suspension of microorganisms, primarily bacteria, to biologically break down organic pollutants in the wastewater. The activated sludge process takes place in an aeration tank, where the wastewater is mixed with the activated sludge and air is supplied. The microorganisms in the activated sludge consume the organic matter in the wastewater as food, oxidizing it to carbon dioxide and water. This process removes the organic pollutants from the wastewater. The air supplied to the aeration tank provides oxygen for the microorganisms to carry out this aerobic respiration and also keeps the sludge in suspension. After the aeration tank, the mixture of treated wastewater and activated sludge flows to a settling tank, also known as a clarifier. Here, the activated sludge settles to the bottom, forming a concentrated sludge layer. A portion of this settled sludge is then recycled back to the aeration tank to maintain a high concentration of microorganisms and ensure continuous treatment. The remaining sludge is removed from the system and processed for disposal. The treated wastewater, now with a reduced level of organic pollutants, is discharged from the settling tank for further treatment or disposal. The key to the activated sludge process is maintaining a healthy and active population of microorganisms that can efficiently remove organic matter from the wastewater.